The objective of this R01 application is to develop an integrated small-animal whole-body photoacoustic- ultrasonic computed tomography system and the associated image reconstruction algorithms for simultaneous high-resolution anatomical and functional imaging with motion tracking. Due to the widespread use of animal models for human disease studies, small-animal whole-body imaging plays an increasingly important role in biomedical research. While much effort has been invested in the development of small-animal imaging systems, each of the available methods possesses significant limitations. Photoacoustic computed tomography (PACT) has recently been recognized as a promising tool for small- animal whole-body imaging. Utilizing the photoacoustic effect, PACT can image intact biological tissues with rich optical absorption contrast at high spatial resolution at tissue depth well beyond the optical diffusion limit (~1 mm). Since optical absorption is sensitive to physiological parameters such as the total concentration and oxygen saturation of hemoglobin, PACT can provide both anatomical and functional imaging. With the aid of functionalized contrast agents (molecular probes), PACT can also permit molecular imaging. Most PACT systems implemented to date assume that the to-be-imaged object possesses uniform acoustic properties. This assumption is strongly violated in whole-body imaging of small animals due to the presence of either thick bones or gas pockets, which possess speed-of-sound and mass density values greatly different from those of the surrounding soft tissues. In addition, the existing PACT systems suffer from significant motion artifacts. Accordingly, there remains an important need for the development of improved small-animal PACT systems and associated image reconstruction methodologies. The proposed integration of PACT and ultrasonic computed tomography (USCT) with motion tracking will bring unique advantages and allow us to overcome the two challenges mentioned above. First, the acoustic property distributions reconstructed by use of USCT will be employed to inform the PACT image reconstruction algorithms and hence improve the whole-body image quality. The ultrasonic contrasts will also complement optical contrasts from PACT for accurate multi-faceted disease assessment. Second, to minimize motion artifacts, respiration and cardiac motions will be monitored during data acquisition for retrospective gating. Therefore, the synergistic fusion of USCT and PACT will provide automatically co-registered anatomical and functional contrasts for comprehensive imaging without using ionizing radiation or exogenous contrast agents. The specific aims of this project are as follows: (1) Develop an integrated whole-body photoacoustic- ultrasonic computed tomography system. (2) Develop image reconstruction algorithms for use with the dual- modality imaging system. (3) Develop retrospectively respiration-gated tomography that minimizes motion artifacts. (4) Test the imaging systems with tissue phantoms and living animals.